Thermoplastic resin films, especially biaxially-oriented polyester films are broadly used as a substrate film in various applications such as magnetic recording materials and packaging materials due to their excellent mechanical properties, electric properties, dimensional stability, transparency, chemical resistance and the like. For the metal can application, the production of white-colored laminated metal sheets by laminating a white film on a metal sheet to provide a clear print on the can surface (for example, JP 2000-177085 A, JP 2001-212918 A and JP H11-262987 A) has been discussed. Since coating is not necessary in that process, the white-colored laminated metal sheets have been proposed as an environment-friendly material which does not use an organic solvent. For the laminated metal sheet application, the improvement of releasability of the content by including wax has been discussed (for example, JP 2001-220453 A, JP 2006-130676 A and JP 2002-302559 A).
Although the films disclosed in JP 2000-177085 A, JP 2001-212918 A and JP H11-262987 A had base coverage property and did not cause a serious problem upon the production of cans in a shallow-drawn shape, when the cans were produced in a deep-drawn shape, they caused a problem in which damage was likely to occur due to the friction with the mold during can production when the surface touched the mold. The films disclosed in JP 2001-220453 A, JP 2006-130676 A and JP 2002-302559 A contained wax. However, it was a design primarily for better releasability of the content and not an appropriate design to provide scratch resistance during production of deep-drawn cans. For the light-blocking tape application, there was a processing drawback of foreign substances such as dust adherent to the film surface when an adhesive layer was coated on the film or when print processing was performed, resulting in a problem of the decrease in quality and yield of the light-blocking tape. On the other hand, coating a releasing layer alone on the film may have decreased the print property. Therefore, a film has been demanded which exhibits releasability before the processing and then acquires suitability for coating and print processing due to the heat caused by the processing such as coating and printing.
Thus, it could be helpful to provide a polyester film used suitably for applications in laminated metal sheets for deep-drawing molding, light-blocking tapes and the like. The second object is to provide a thin polyester film but having good base coverage property and good processing suitability at a high temperature, as well as to provide a white film with good molding processability, print property and coverage property as well as excellent processability at a high temperature and excellent detachability during a molding processing.